1. Field of the Invention
This invention relates generally to methods of patient diagnosis through processing of electrocardiogram (ECG) data, and particularly to methods and apparatus for evaluating the autonomic nervous system control of the heart through novel processing of ECG data.
2. Description of Background Art
The cardiovascular system responds to various demands of the body under control of the autonomic nervous system, which consists of the parasympathetic and sympathetic limbs. Certain pathologic conditions such as diabetic autonomic neuropathy and post-myocardial infarction can impair the autonomic control of the heart, which could result in various arrhythmias such as ventricular fibrillation, ventricular tachycardia, sinus bradycardia, atrioventricular block, and sudden asystole, among others. An imbalance between the sympathetic autonomic control and the parasympathetic autonomic control may predispose a patient to sudden cardiac death.
A conventional practice for evaluation of the autonomic nervous system control of the heart is the study of heart period variability (in both the time and frequency domains) under various conditions of autonomic stimulation including deep breathing, exercise, rest, mental and pharmacological stress testing, and various other autonomic maneuvers. However, clinical data has shown that heart period variability is predominantly representative of parasympathetic tone only, and that the ratio of low to high frequency spectral power of heart rate variability is not always reliable as a measure of sympathetic autonomic activity. As a result, analysis of heart period variability alone cannot detect adequately imbalances between the parasympathetic and sympathetic autonomic controls of the heart. Furthermore, heart rate variability cannot directly reflect ventricular response to autonomic stimuli, which may be a critical determinant of the susceptibility of a patient to ventricular arrhythmias and sudden cardiac death.